ANSI-C program: ASAR_IM_proc.c
NAME
ASAR_IM_proc - Generates MSP sensor parameter and processing
parameter files from Level 0 (raw) ASAR Image Mode data. This
program also reformats the raw SAR signal data to be compatible
with the MSP. The entire level 0 data set is converted to
8-bit I/Q unsigned binary complex samples.
SYNOPSIS
ASAR_IM_proc <L0>
<INS> <SAR_par> <PROC_par> <raw>
<ant_gain> [loff] [nl]
| L0 | (input) Level 0 Image Mode ASAR data file
(ASA_IM_0P) |
| INS | (input) ASAR instrument characterization file (ASA_INS_AX) |
| SAR_par | (output) MSP SAR sensor parameter file |
| PROC_par |
(output) MSP processing
parameter file, (example p19969421.slc.par) |
| raw | (output) byte aligned 8-bit I/Q raw data |
| ant_gain |
(input) antenna pattern
file name (derived using program ASAR_XCA) |
| loff |
offset lines to first line
to extract (default = 0, enter - for default) |
| nl | number of lines to extract
(default = to end of input, enter - for default) |
EXAMPLE
ASAR_IM_proc
ASA_IM__0CNPDK20030507_092426_000000152016_00122_06188_0390.N1
ASA_INS_AXVIEC20030424_154116_20030428_000000_20031231_235959
20030507.is3.par p20030507.slc.par 20030507.raw
is3.ant
DESCRIPTION
ASAR_IM_proc reads the
ASAR Level 0 data files and generates the MSP processing
parameter (p*.slc.par) and sensor parameter files along with
reformatting the raw data. Level 0 files have names that begin
with ASA_IM__0.... These
files contain both raw data and meta data describing when the
data were acquired and the imaging mode (IS1 - IS7).
Detailed information on the raw data Level 0 format is contaned
in the ASAR Product Handbook available on-line at http://envisat.esa.int.
The ASAR imaging mode data are stored as records in the file
with each record corresponding to an echo. Each echo
consists of a header consisting of timing and error information
and radar parameters followed by a set of 64-byte packets
containing the echo samples. These echo samples are stored using
block floating point quantization (FBAQ) that effectively
increases the dynamic range of the data to 8 bits from the 4 bits
used to encode the data in the Level 0 data. Each packet
consists of 63 complex samples (4-bit I/4-bit Q) and a single
byte that is an index into a set of lookup tables. These lookup
tables map the 4-bit values to the corresponding 8-bit
values.
The set of 256 look up tables needed to generate decompressed
samples and other ASAR calibration and system parameters are
stored in the Instrument Characterization File (ASA_INS_AXVIEC...). These files
must be downloaded from ESA and the correct file selected for the
date that the data were aquired. The ESA website to
download both the INS files and the external characterization
file (ASA_XCA...) is
http://envisat.esa.int/services/auxiliary_data/asar
The external characterization file is required to extract the
antenna pattern as measured on the ground and is used to
compensate for the antenna gain across the swath in the SAR
processing. The program in the ISP called ASAR_XCA and is used to
extract the appropriate antenna pattern (the ant_gain value on the command
line).
The ASAR instrument shifts the echo digitization window
approximately every 15 seconds along-track. ASAR_IM_proc
initially scans the entire data set to determine the maximum
shift from start to finish. and shifts the data in the output
such that the starting time for all echoes is the same for the
entire raw data set.. No samples are lost in this process because
the data are extended to accomodate the range of the shift. Every
echo raw data file is decompressed and written to the output
raw file specified on the
command line.
You also have the possibility to extract only a subset of the echoes from the raw data set. This is done by specifing the offset number of echoes to the first echo to extract loff, and the number of echoes to extract nl. This capability may be useful in the case of very large raw data sets.
SEE ALSO
Users Guide, typedef_MSP.h, SAR Sensor Parameter File, MSP Processing Parameter File, SAR raw data
(<YYYYMMDD>.raw).